Scientists ID possible Tunguska crater

Was it a bird, a plane or a big sodding rock?

The largest earth "impact" of recent times, the Tunguska event, might have left a crater after all. The impact levelled more than 2,000 square kilometres of forest in Siberia, but is thought to have been caused by an asteroid or comet exploding in Earth's atmosphere since no crater marking the impact has ever been found. Now researchers at the University of Bologna in Italy think they might have found a scar in the surface of the earth left by the 1908 astro-intrusion.

The team of researchers has identified a lake near the epicentre of the blast whose shape is consistent, they say, with an impact crater. They speculate that a small chunk of debris from the main explosion could have hit the ground, creating the hollow.

Lead researcher professor Giuseppe Longo says he concluded the lake was sitting in a crater by a process of elimination. He told BBC News: "We have no positive proof this is an impact crater, but we were able to exclude some other hypotheses, and this led us to our conclusion."

He says that the lake-bed's geology is unusual. It has a funnel shape that is unlike that of any of its neighbours. After probing the bottom with geophysics equipment, the team also discovered an anomalous feature about 10 metres down. They suggest this could be either compacted lake sediment, or debris left over from a piece of space junk.

But the scientific community is sceptical of professor Longo's hypothesis. Dr Gareth Collins, a research associate at Imperial College London, told the BBC: "The impact-cratering community does not accept structures as craters unless there is evidence of high temperatures and high pressures. That requires evidence of rocks that have been melted or rocks that have been ground up by the impact."

The Tunguska event took place on June 30, 1908. The blast, which had the energy of about 1,000 Hiroshima-sized nukes, was so bright it was even seen in the London skies. No debris from the explosion has ever been found, much less any traces of an impact.

Dr Collins says the lake Longo and his team have studied is not consistent with what we already know about Tunguska. The angle of impact is wrong, any fragments surviving the initial blast would be moving too slowly, and there are trees older than 100 years still standing near the lake, making it unlikely anything from space crashed nearby.

But the Bologna researchers are undeterred. They plan to return to the site and dig into the lake bed to find out whether or not what is buried below the surface once roamed the outer reaches of our solar system. ®